Buried beneath 20 kilometers of ice, the subsurface ocean of Enceladus—one of Saturn’s moons—appears to be churning with currents akin to those on Earth.

The theory, derived from the shape of Enceladus’s ice shell, challenges the current thinking that the moon’s global ocean is homogenous, apart from some vertical mixing driven by the warmth of the moon’s core.

Enceladus, a tiny frozen ball about 500 kilometers in diameter (about 1/7th the diameter of Earth’s moon), is the sixth largest moon of Saturn. Despite its small size, Enceladus attracted the attention of scientists in 2014 when a flyby of the Cassini spacecraft discovered evidence of its large subsurface ocean and sampled water from geyser-like eruptions that occur through fissures in the ice at the south pole. It is one of the few locations in the solar system with liquid water (another is Jupiter’s moon Europa), making it a target of interest for astrobiologists searching for signs of life.

The ocean on Enceladus is almost entirely unlike Earth’s. Earth’s ocean is relatively shallow (an average of 3.6 km deep), covers three-quarters of the planet’s surface, is warmer at the top from the sun’s rays and colder in the depths near the seafloor, and has currents that are affected by wind; Enceladus, meanwhile, appears to have a globe-spanning and completely subsurface ocean that is at least 30 km deep and is cooled at the top near the ice shell and warmed at the bottom by heat from the moon’s core.

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